Electronic structure of CsPbBr3−xClx perovskites: synthesis, experimental characterization, and DFT simulations

Post-hot-cast annealing deposition of perovskite films with infused multifunctional organic molecules to enhance the performance of large-area light-emitting devices

All-inorganic perovskites show great promise as an emission layer in perovskite light-emitting diodes (PeLEDs) owing to their easy solution processing, low manufacturing cost, and excellent optoelectronic properties. However, there is still an immense performance gap from small-area devices to large-area PeLED devices. The inhomogeneity of large-area high-quality perovskite films inevitably leads to vast defects and electroluminescence performance losses. Herein, a post-hot-cast annealing deposition scheme and the introduction of the multifunctional molecule 2-amino-1,3-propanediol (APDO) were proposed to regulate the crystallization of the perovskite film. As a result, uniform APDO:CsPbBr2.5Cl0.5 perovskite films with high crystallinity and lower defect density were deposited by post-hot-cast annealing. A decent maximum brightness of 2659 cd m-2 was achieved for the large-area cyan PeLEDs with an emitting area of 400 mm2.

Structural and chemical properties of anion exchanged CsPb(Br(1-x)Clx)3heterostructured perovskite nanowires imaged by nanofocused x-rays

Over the last years metal halide perovskites have demonstrated remarkable potential for integration in light emitting devices. Heterostructures allow for tunable bandgap depending on the local anion composition, crucial for optoelectronic devices, but local structural effects of anion exchange in single crystals is not fully understood. Here, we investigate how the anion exchange of CsPbBr3nanowires fully and locally exposed to HCl vapor affects the local crystal structure, using nanofocused x-rays. We study the nanoscale composition and crystal structure as function of HCl exposure time and demonstrate the correlation of anion exchange with changes in the lattice parameter. The local composition was measured by x-ray fluorescence and x-ray diffraction, with general agreement of both methods but with much less variation using latter. The heterostructured nanowires exhibit unintentional gradients in composition, both axially and radially. Ferroelastic domains are observed for all HCl exposure times, and the magnitude of the lattice tilt at the domain walls scales with the Cl concentration.

Photoinduced Transition from Quasi-Two-Dimensional Ruddlesden-Popper to Three-Dimensional Halide Perovskites for the Optical Writing of Multicolor and Light-Erasable Images

Optical data storage, information encryption, and security labeling technologies require materials that exhibit local, pronounced, and diverse modifications of their structure-dependent optical properties under external excitation. Herein, we propose and develop a novel platform relying on lead halide Ruddlesden-Popper phases that undergo a light-induced transition toward bulk perovskite and employ this phenomenon for the direct optical writing of multicolor patterns. This transition causes the weakening of quantum confinement and hence a reduction in the band gap. To extend the color gamut of photoluminescence, we use mixed-halide compositions that exhibit photoinduced halide segregation. The emission of the films can be tuned across the range of 450-600 nm. Laser irradiation provides high-resolution direct writing, whereas continuous-wave ultraviolet exposure is suitable for recording on larger scales. The luminescent images created on such films can be erased during the visualization process. This makes the proposed writing/erasing platform suitable for the manufacturing of optical data storage devices and light-erasable security labels.

Mechanistic Insights into the Photoluminescence Enhancement in Surface Ligand Modified CsPbBr3 Perovskite Nanocrystals

We report a mechanistic study of the photoluminescence (PL) enhancement in CsPbBr3 perovskite nanocrystals (PNCs) induced by organic/inorganic hybrid ligand engineering. Compared to the as-synthesized oleic acid-oleylamine modified PNCs, the tributylphosphine oxide-CaBr2 modified PNCs can achieve a better passivation effect due to strong P═O-Pb coordination and Br-vacancy remedy, resulting in enhanced PL efficiency. We employ steady-state/time-resolved/temperature-dependent PL and fluence/polarization-dependent ultrafast transient absorption spectroscopy to obtain a mechanistic understanding of such an enhancement effect from both nonradiative and radiative perspectives. As for the dominating nonradiative recombination suppression, we quantitatively evaluate the contributions from channels of exciton dissociation and exciton trapping, which are connected to exciton binding energy and activation energy of exciton trapping to surface defect-induced trap states, respectively. We also look into the radiative recombination enhancement, which is likely due to the increase in electron-hole overlap of photogenerated excitons induced by slight Ca-doping. These mechanistic insights would be of guiding value for perovskite-based light-emitting applications.

Nanoscale X-ray Imaging of Composition and Ferroelastic Domains in Heterostructured Perovskite Nanowires: Implications for Optoelectronic Devices

Metal halide perovskites (MHPs) have garnered significant interest as promising candidates for nanoscale optoelectronic applications due to their excellent optical properties. Axially heterostructured CsPbBr3-CsPb(Br(1-x)Clx)3 nanowires can be produced by localized anion exchange of pregrown CsPbBr3 nanowires. However, characterizing such heterostructures with sufficient strain and real space resolution is challenging. Here, we use nanofocused scanning X-ray diffraction (XRD) and X-ray fluorescence (XRF) with a 60 nm beam to investigate a heterostructured MHP nanowire as well as a reference CsPbBr3 nanowire. The nano-XRD approach gives spatially resolved maps of composition, lattice spacing, and lattice tilt. Both the reference and exchanged nanowire show signs of diverse types of ferroelastic domains, as revealed by the tilt maps. The chlorinated segment shows an average Cl composition of x = 66 and x = 70% as measured by XRD and XRF, respectively. The XRD measurements give a much more consistent result than the XRF ones. These findings are consistent with photoluminescence measurements, showing x = 73%. The nominally unexchanged segment also has a small concentration of Cl, as observed with all three methods, which we attribute to diffusion after processing. These results highlight the need to prevent such unwanted processes in order to fabricate optoelectronic devices based on MHP heterostructures.

Electronic Structure and Optical Properties of Inorganic Pm3m and Pnma CsPbX3 (X = Cl, Br, I) Perovskite: A Theoretical Understanding from Density Functional Theory Calculations

In this study, we investigated the optoelectronic properties of cubic (Pm3m) and orthorhombic (Pnma) CsPbX3 (X = I, Br, and Cl). We utilized the full potential linear augmented plane wave method, which is implemented in the WIEN2k code, to facilitate the investigation. Different exchange potentials were used to analyze the optoelectronic behavior using the available density functional theory methods. Our findings revealed that CsPbX3 perovskites display direct band gaps at the R and Г points for cubic (Pm3m) and orthorhombic (Pnma) structures, respectively. Among the exchange potentials, the mBJ-GGA method provided the most accurate results. These outcomes concurred with the experimental results. In both Pm3m and Pnma structures, interesting changes were observed when iodide (I) was replaced with bromine (Br) and then chlorine (Cl). The direct band gap at the R and Г points shifted to higher energy levels. Similarly, when I was replaced with Br and Cl, there was a noticeable decrease in the absorption coefficient, dielectric constants, refractive index, and reflectivity, in addition to a band gap shift to higher energy levels.

Giant and Tunable Excitonic Optical Anisotropy in Single-Crystal Halide Perovskites

During the last years, giant optical anisotropy has demonstrated its paramount importance for light manipulation. In spite of recent advances in the field, the achievement of continuous tunability of optical anisotropy remains an outstanding challenge. Here, we present a solution to the problem through the chemical alteration of halogen atoms in single-crystal halide perovskites. As a result, we manage to continually modify the optical anisotropy by 0.14. We also discover that the halide perovskite can demonstrate optical anisotropy up to 0.6 in the visible range─the largest value among non-van der Waals materials. Moreover, our results reveal that this anisotropy could be in-plane and out-of-plane depending on perovskite shape─rectangular and square. As a practical demonstration, we have created perovskite anisotropic nanowaveguides and shown a significant impact of anisotropy on high-order guiding modes. These findings pave the way for halide perovskites as a next-generation platform for tunable anisotropic photonics.

Universal scaling laws for charge-carrier interactions with quantum confinement in lead-halide perovskites

Lead halide perovskites open great prospects for optoelectronics and a wealth of potential applications in quantum optical and spin-based technologies. Precise knowledge of the fundamental optical and spin properties of charge-carrier complexes at the origin of their luminescence is crucial in view of the development of these applications. On nearly bulk Cesium-Lead-Bromide single perovskite nanocrystals, which are the test bench materials for next-generation devices as well as theoretical modeling, we perform low temperature magneto-optical spectroscopy to reveal their entire band-edge exciton fine structure and charge-complex binding energies. We demonstrate that the ground exciton state is dark and lays several millielectronvolts below the lowest bright exciton sublevels, which settles the debate on the bright-dark exciton level ordering in these materials. More importantly, combining these results with spectroscopic measurements on various perovskite nanocrystal compounds, we show evidence for universal scaling laws relating the exciton fine structure splitting, the trion and biexciton binding energies to the band-edge exciton energy in lead-halide perovskite nanostructures, regardless of their chemical composition. These scaling laws solely based on quantum confinement effects and dimensionless energies offer a general predictive picture for the interaction energies within charge-carrier complexes photo-generated in these emerging semiconductor nanostructures.

Smart thermally responsive perovskite materials: Thermo-chromic application and density function theory calculation

With the continuous improvement of human's requirements for temperature control suitable for living, the energy consumption of electrical appliances such as air conditioners has become a major challenge in traditional architectural design. Generally, most of the solar energy passes through the glass to enter and exit the building, but the traditional glass can hardly control the light and heat energy, causing the indoor temperature to change dramatically with the environment. Therefore, it is more urgent to develop green and efficient smart windows. Perovskite is a temperature-adaptive material, which has the ability of phase transition and can adjust its band gap for thermochromic applications. In this work, we study the perovskite-based thermochromic smart window. As a new application of perovskite, a number of experiments have been carried out. However, there is still a lack of theoretical analysis on phase transition mechanisms and crystal structure prediction. Density functional theory (DFT) calculation is the most useful tool in optoelectronics, especially for perovskite crystal. Here, we extracted typical cases from published literature for analysis and comparison and summarized the crystal structure, electronic structure stability, interface engineering, and thermal characteristics employing DFT calculation We believe this work will pave the way for DFT application for the study of thermochromic perovskite.

Phase stability and electronic structure of CsPbCl3 under hydrostatic stress and anion substitution

Perovskites based on CsPbX₃ (X = Cl, Br, I) have promising applications in solar cells, light-emitting diodes, and photodetectors. In this paper, the phase stability of inorganic metal halide perovskite CsPbCl₃ under hydrostatic pressure and anion substitution is studied using density functional theory (DFT), and this modification is explained by the interaction of the octahedrons and transformation of the bond-orbital coupling. In addition, two space groups, P4/mbm and Amm2, which are stable under stress, are subjected to anion substitution; then, the structural stability and band gap change of CsPbCl_3-yX_y (X = Br, I; y = 0, 1, 2, 3) are analyzed after applying stress; finally, the electronic structures and optical properties of the six most stable components are presented. The effect of stress and anions on the components' optoelectronic properties is closely linked with the crystal's structural alteration mechanism. These results show that stress and anion modulation can significantly change the optoelectronic properties of materials, which make these materials have broad application prospects. Furthermore, stress can be used as an effective tool for screening the most stable material structure.

Perovskite Nanowire Laser for Hydrogen Chloride Gas Sensing

Detection of hazardous volatile organic and inorganic species is a crucial task for addressing human safety in the chemical industry. Among these species, there are hydrogen halides (HX, X = Cl, Br, I) vastly exploited in numerous technological processes. Therefore, the development of a cost-effective, highly sensitive detector selective to any HX gas is of particular interest. Herein, we demonstrate the optical detection of hydrogen chloride gas with solution-processed halide perovskite nanowire lasers grown on a nanostructured alumina substrate. An anion exchange reaction between a CsPbBr3 nanowire and vaporized HCl molecules results in the formation of a structure consisting of a bromide core and thin mixed-halide CsPb(Cl,Br)3 shell. The shell has a lower refractive index than the core does. Therefore, the formation and further expansion of the shell reduce the field confinement for experimentally observed laser modes and provokes an increase in their frequency. This phenomenon is confirmed by the coherency of the data derived from XPS spectroscopy, EDX analysis, in situ XRD experiments, HRTEM images, and fluorescent microspectroscopy, as well as numerical modeling for Cl- ion diffusion and the shell-thickness-dependent spectral position of eigenmodes in a core-shell perovskite nanowire. The revealed optical response allows the detection of HCl molecules in the 5-500 ppm range. The observed spectral tunability of the perovskite nanowire lasers can be employed not only for sensing but also for their precise spectral tuning.

Unusual isostructural Br/I substitution effect on the crystal structure and optical properties of hybrid halobismuthates

The gradual change of the optical properties of solid solutions by variation of the halogen composition is shown.

Inorganic Copper-Based Halide Perovskite for Efficient Photocatalytic CO2 Reduction

In view of the toxicity of the Pb element, exploring eco-friendly Pb-free halide perovskites with excellent photoelectric properties is of great research and practical application significance. Herein, copper-based halide perovskite CsCuCl₃ and the corresponding Br^--substituted sample (CsCuCl₂Br) are designed and explored as the catalysts for photocatalytic CO₂ reduction for the first time. A facile antisolvent recrystallization process with pre-prepared single crystals as the precursor is employed to controllably synthesize CsCuCl₃ and CsCuCl₂Br microcrystals (MCs). The electronic structure and charge transfer property analysis by theoretical and experimental investigation reveal that CsCuCl₃ possesses a satisfying bandgap (1.92 eV) and conduction band minimum (CBM) to harvest the sunlight and drive the conversion of CO₂ to CH₄ and CO. The Br^- substitution can not only narrow the bandgap but also facilitate the transportation of charge carriers. Thus, a total electron consumption rate of 44.71 μmol g^-1 h^-1 is achieved for CsCuCl₂Br MCs, which is much better than that of same-sized CsPbBr₃ microcrystals or even better than many perovskite nanocrystal photocatalysts. This study suggests that Cu-based perovskites can serve as promising candidates for artificial photosynthesis or other photocatalytic applications, which may propose a new thought to construct lead-free, low-cost photocatalysts.

Stable and Bright Electroluminescent Devices utilizing Emissive 0D Perovskite Nanocrystals Incorporated in a 3D CsPbBr3 Matrix

The 0D cesium lead halide perovskite Cs₄ PbBr₆ has drawn remarkable interest due to its highly efficient robust green emission compared to its 3D CsPbBr₃ counterpart. However, seizing the advantages of the superior photoluminescence properties for practical light-emitting devices remains elusive. To date, Cs₄ PbBr₆ has been employed only as a higher-bandgap nonluminescent matrix to passivate or provide quantum/dielectric confinement to CsPbBr₃ in light-emitting devices and to enhance its photo-/thermal/environmental stability. To resolve this disparity, a novel solvent engineering method to incorporate highly luminescent 0D Cs₄ PbBr₆ nanocrystals (perovskite nanocrystals (PNCs)) into a 3D CsPbBr₃ film, forming the active emissive layer in single-layer perovskite light-emitting electrochemical cells (PeLECs) is designed. A dramatic increase of the maximum external quantum efficiency and luminance from 2.7% and 6050 cd m^-2 for a 3D-only PeLEC to 8.3% and 11 200 cd m^-2 for a 3D-0D PNC device with only 7% by weight of 0D PNCs is observed. The majority of this increase is driven by the efficient inherent emission of the 0D PNCs, while the concomitant morphology improvement also contributes to reduced leakage current, reduced hysteresis, and enhanced operational lifetime (half-life of 129 h), making this one of the best-performing LECs reported to date.

Amplified Spontaneous Emission from Thermally Evaporated High-Quality Thin Films of CsPb(Br1-xYx)3 (Y = I, Cl) Perovskites

As a wavelength-tunable lasing material, perovskites are now generating a lot of scientific attention. Conventional solution-processed CsPbX₃ perovskite films sometimes suffer unavoidable pinhole defects and poor surface morphology, severely limiting their performance on amplified spontaneous emission (ASE) and lasing application. Herein, a thermal evaporation approach is explored in our work to achieve a uniform and high-coverage CsPb(Br_1-xY_x)₃ (Y = I, Cl) perovskites polycrystalline thin film. The ASE of these films was studied using a picosecond laser system. The ASE profile increases rapidly over the narrow peak in relation to the laser pump intensity, confirming the development of stimulated emission. ASE began when the energy density threshold was reached and ranged between 25 and 170 μJ/cm² per pulse for perovskite materials when replacing I with Br and then Cl. This work emphasizes the notable optical properties of high-quality perovskite thin films, leading to possible accessible uses in optoelectronic applications.

Effects of Electron-Phonon Coupling and Spin-Spin Coupling on the Photoluminescence of Low-Dimensional Metal Halides

Low-dimensional metal halides (LDMHs), as a derivative of three-dimensional lead halide perovskites, have attracted much attention because of their unique crystal structures and fascinating photonic properties. The simple synthesis and rich photonic properties of LDMHs make them striking candidates for the development of lighting, photodetectors, biological imaging, etc. Although many novel LDMHs have been achieved with strong electron-phonon coupling related to their self-trapped excitons (STEs) and excellent optical responses, transition-metal halides or doped halides have not been covered in regard to their rich spin characteristics. In this Perspective, we aim to deeply understand the role of electron-phonon coupling and STEs with magnetic coupling effects in regulating the optical properties of LDMHs and try to provide a novel way or a series of novel systems for the realization of next-generation high-performance luminescent materials with spin-coupling-involved photonics. Finally, an outlook toward potential challenges and applications of such ionic semiconducting LDMHs is also presented.

Spatially resolved fluorescence of caesium lead halide perovskite supercrystals reveals quasi-atomic behavior of nanocrystals

We correlate spatially resolved fluorescence (-lifetime) measurements with X-ray nanodiffraction to reveal surface defects in supercrystals of self-assembled cesium lead halide perovskite nanocrystals and study their effect on the fluorescence properties. Upon comparison with density functional modeling, we show that a loss in structural coherence, an increasing atomic misalignment between adjacent nanocrystals, and growing compressive strain near the surface of the supercrystal are responsible for the observed fluorescence blueshift and decreased fluorescence lifetimes. Such surface defect-related optical properties extend the frequently assumed analogy between atoms and nanocrystals as so-called quasi-atoms. Our results emphasize the importance of minimizing strain during the self-assembly of perovskite nanocrystals into supercrystals for lighting application such as superfluorescent emitters.

By utilizing spatially resolved fluorescence (-lifetime) measurements and high precision X-ray nanodiffraction, the authors correlate the influence of structural misalignment and fluorescence (-lifetime) properties of all-inorganic CsPbX₃ (X^– = Br^–, Cl^–) perovskite superlattices.

A comparative study of the mechanical stability, electronic, optical and photocatalytic properties of CsPbX3 (X = Cl, Br, I) by DFT calculations for optoelectronic applications

Organic free Cs-based perovskite materials are potential candidates for electronic and optoelectronic applications. A systematic comparative study of the mechanical, electronic, optical, and photocatalytic properties of CsPbX₃ (X = Cl, Br, I) was conducted using density functional theory to compare the applicability of these materials in optoelectronic, photocatalytic, and photovoltaic (PV) devices. We calculated structural and elastic properties to determine the better agreement of damage-tolerance and electronic and optical responses for suitable device applications. Optimized lattice parameters and elastic constants showed excellent agreement with the experimental data whereas some properties were found to be much better than other theoretical reports. CsPbBr₃ is thermodynamically more stable and more ductile compared to the other two perovskites. The hydrostatic pressure dependent mechanical stability showed that CsPbCl₃ and CsPbBr₃ sustained stability under low applied pressure, whereas the stability of CsPbI₃ was very high. The electronic band gap calculations showed that CsPbCl₃, CsPbBr_3, and CsPbI₃ are suitable for green, orange, and red emissions of optical spectra owing to the proper electronic band gaps. CsPbI₃ can be shown as the best photocatalyst for the hydrogen evolution reaction and CsPbBr₃ is the most stable photocatalyst due to its nearly balanced oxidation and reduction potentials, but CaPbCl₃ is better for O₂ production. The density of states and other optical properties have been reported in this study. Thus, our findings would be beneficial for experimental studies and can open a new window for efficient electronic, optoelectronic, and hydrogen production along with the biodegradation of polluted and waste materials.

Organic free Cs-based perovskite materials are potential candidates for electronic and optoelectronic applications.

Ultra-violet to visible band gap engineering of cubic halide KCaCl3 perovskite under pressure for optoelectronic applications: insights from DFT

Density functional theory is utilized to explore the effects of hydrostatic pressure on the structural, electrical, optical, and mechanical properties of cubic halide perovskite KCaCl₃ throughout this study. The interatomic distance is decreased due to the pressure effect, which dramatically lowers the lattice constant and unit cell volume of this perovskite. Under pressure, the electronic band gap shrinks from the ultra-violet to visible region, making it easier to move electrons from the valence band to the conduction band, which improves optoelectronic device efficiency. Furthermore, the band gap nature is switched from indirect to direct around 40 GPa pressure, which is more suitable for a material to be exploited in optoelectronic applications. The use of KCaCl₃ in microelectronics, integrated circuits, QLED, OLED, solar cells, waveguides, solar heat reduction materials, and surgical instruments has been suggested through deep optical analysis. The use of external hydrostatic pressure has a considerable impact on the mechanical properties of this material, making it more ductile and anisotropic.

Electronic and optical properties of bulk and surface of CsPbBr3 inorganic halide perovskite a first principles DFT 1/2 approach

This work aims to test the effectiveness of newly developed DFT-1/2 functional in calculating the electronic and optical properties of inorganic lead halide perovskites CsPbBr3. Herein, from DFT-1/2 we have obtained the direct band gap of 2.36 eV and 3.82 eV for orthorhombic bulk and 001-surface, respectively. The calculated energy band gap is in qualitative agreement with the experimental findings. The bandgap of ultra-thin film of CsPbBr3 is found to be 3.82 eV, which is more than the expected range 1.23-3.10 eV. However, we have found that the bandgap can be reduced by increasing the surface thickness. Thus, the system under investigation looks promising for optoelectronic and photocatalysis applications, due to the bandgap matching and high optical absorption in UV-Vis (Ultra violet and visible spectrum) range of electro-magnetic(em) radiation.

Crystal-Plane Controlled Spontaneous Polarization of Inorganic Perovskite toward Boosting Triboelectric Surface Charge Density

Triboelectric generators (TENGs) have been extensively studied as a new energy for low cost and the universally applicable prospect. Meanwhile, perovskites have been applied in TENG and show a good performance in view of high carrier mobility, long life and dielectric properties. The asymmetry structure of the orthogonal phase CsPbBr₃ perovskite endows it with ferroelectric property and induces the misalignment of the positive and negative charge centers. Herein, the surface energy of halogen doped inorganic CsPbX₃ (X = Cl^-, Br^-) perovskites are theoretically investigated by density functional theory (DFT) calculation, the crystal polarizability of pristine CsPbBr₃ is improved from 0.47 Ry a.u. to 0.52 Ry a.u. (CsPbCl₃), indicating the polarizability of CsPbCl₃ is higher than CsPbBr₃. In addition, the build-in electric field (E_build-in) of perovskite materials can be enhanced by the spontaneous polarization and the aligned dipoles in the E_build-in could further improve the tribo-electrostatic electric field by retaining more triboelectric surface charges. In the end, CsPbCl₃ achieved a power of 3.06 W m^-2 compared to the power of 1.34 W m^-2 of CsPbBr₃. This work focuses on the regulation of crystal planes using spontaneous polarization of perovskite toward achieving a high built-in electric field for enhancing triboelectric surface charge density.

Structural, Electronic, and Optical Properties of CsPb(Br1-xClx)3 Perovskite: First-Principles Study with PBE-GGA and mBJ-GGA Methods

The effect of halide composition on the structural, electronic, and optical properties of CsPb(Br1-xClx)3 perovskite was investigated in this study. When the chloride (Cl) content of x was increased, the unit cell volume decreased with a linear function. Theoretical X-ray diffraction analyses showed that the peak (at 2θ = 30.4°) shifts to a larger angle (at 2θ = 31.9°) when the average fraction of the incorporated Cl increased. The energy bandgap (Eg) was observed to increase with the increase in Cl concentration. For x = 0.00, 0.25, 0.33, 0.50, 0.66, 0.75, and 1.00, the Eg values calculated using the Perdew-Burke-Ernzerhof potential were between 1.53 and 1.93 eV, while those calculated using the modified Becke-Johnson generalized gradient approximation (mBJ-GGA) potential were between 2.23 and 2.90 eV. The Eg calculated using the mBJ-GGA method best matched the experimental values reported. The effective masses decreased with a concentration increase of Cl to 0.33 and then increased with a further increase in the concentration of Cl. Calculated photoabsorption coefficients show a blue shift of absorption at higher Cl content. The calculations indicate that CsPb(Br1-xClx)3 perovskite could be used in optical and optoelectronic devices by partly replacing bromide with chloride.

Suppression of Electric Field-Induced Segregation in Sky-Blue Perovskite Light-Emitting Electrochemical Cells

Inexpensive perovskite light-emitting devices fabricated by a simple wet chemical approach have recently demonstrated very prospective characteristics such as narrowband emission, low turn-on bias, high brightness, and high external quantum efficiency of electroluminescence, and have presented a good alternative to well-established technology of epitaxially grown III-V semiconducting alloys. Engineering of highly efficient perovskite light-emitting devices emitting green, red, and near-infrared light has been demonstrated in numerous reports and has faced no major fundamental limitations. On the contrary, the devices emitting blue light, in particular, based on 3D mixed-halide perovskites, suffer from electric field-induced phase separation (segregation). This crystal lattice defect-mediated phenomenon results in an undesirable color change of electroluminescence. Here we report a novel approach towards the suppression of the segregation in single-layer perovskite light-emitting electrochemical cells. Co-crystallization of direct band gap CsPb(Cl,Br)3 and indirect band gap Cs4Pb(Cl,Br)6 phases in the presence of poly(ethylene oxide) during a thin film deposition affords passivation of surface defect states and an increase in the density of photoexcited charge carriers in CsPb(Cl,Br)3 grains. Furthermore, the hexahalide phase prevents the dissociation of the emissive grains in the strong electric field during the device operation. Entirely resistant to 5.7 × 106 V·m−1 electric field-driven segregation light-emitting electrochemical cell exhibits stable emission at wavelength 479 nm with maximum external quantum efficiency 0.7%, maximum brightness 47 cd·m−2, and turn-on bias of 2.5 V.

Single-emissive-layer all-perovskite white light-emitting diodes employing segregated mixed halide perovskite crystals

Metal halide perovskites have demonstrated impressive properties for achieving efficient monochromatic light-emitting diodes. However, the development of white perovskite light-emitting diodes (PeLEDs) remains a big challenge. Here, we demonstrate a single-emissive-layer all-perovskite white PeLED using a mixed halide perovskite film as the emissive layer. The perovskite film consists of separated mixed halide perovskite phases with blue and red emissions, which are beneficial for suppressing halide anion exchange and preventing charge transfer. As a result, the white PeLED shows balanced white light emission with Commission Internationale de L'Eclairage coordinates of (0.33, 0.33). In addition, we find that the achievement of white light emission from mixed halide perovskites strongly depends on effective modulation of the halide salt precursors, especially lead bromide and benzamidine hydrochloride in our case. Our work provides very useful guidelines for realizing single-emissive-layer all-perovskite white PeLEDs based on mixed halide perovskites, which will spur the development of high-performance white PeLEDs.

Density Functional Study of Cubic, Tetragonal, and Orthorhombic CsPbBr3 Perovskite

Cesium lead bromide (CsPbBr3) perovskite has recently gained significance owing to its rapidly increasing performance when used for light-emitting devices. In this study, we used density functional theory to determine the structural, electronic, and optical properties of the cubic, tetragonal, and orthorhombic temperature-dependent phases of CsPbBr3 perovskite using the full-potential linear augmented plane wave method. The electronic properties of CsPbBr3 perovskite have been investigated by evaluating their changes upon exerting spin-orbit coupling (SOC). The following exchange potentials were used: the local density approximation (LDA), Perdew-Burke-Ernzerhof generalized gradient approximation (PBE-GGA), Engel-Vosko GGA (EV-GGA), Perdew-Burke-Ernzerhof GGA revised for solids (PBEsol-GGA), modified Becke-Johnson GGA (mBJ-GGA), new modified Becke-Johnson GGA (nmBJ-GGA), and unmodified Becke-Johnson GGA (umBJ-GGA). Our band structure results indicated that the cubic, tetragonal, and orthorhombic phases have direct energy bandgaps. By including the SOC effect in the calculations, the bandgaps computed with mBJ-GGA and nmBJ-GGA were found to be in good agreement with the experimental results. Additionally, despite the large variations in their lattice constants, the three CsPbBr3 phases possessed similar optical properties. These results demonstrate a wide temperature range of operation for CsPbBr3.

Perovskite hetero-anionic-sublattice interfaces for optoelectronics and nonconventional electronics

The perovskite structure provides a versatile framework for functional materials and their high-quality heteroepitaxial interfaces. Perovskite halides (PH) have attracted intense interest for their application in optoelectronics. Oxides are another major class of perovskites that are widely used in fuel cells, nonconventional electronics and electrochemistry. Interfacing different perovskite oxides (POs) has led to a multitude of fascinating discoveries. By introducing anionic degree of freedom, we expect that perovskite hetero-anionic-sublattice interfaces can provide a new platform for emergent phenomena that may or may not have homo-oxygen-sublattice interface analogues. In this work, we investigate the interfaces between the all-inorganic PH CsPbBr₃, the emerging double perovskite halide (dPH) Cs₂TiBr₆ and various common POs. Based on the band alignment properties, these POs are considered to be suitable carrier transport materials (CTMs) for CsPbBr₃ and Cs₂TiBr₆ in either light-harvesting or light-emitting devices. In addition, these perovskite hetero-anionic-sublattice interfaces are found to be defect- and dangling bond-free due to compatible crystal lattices, making POs potentially outperform conventional binary transition-metal-oxide and organic CTMs. Besides optoelectronics, the potential of perovskite hetero-anionic-sublattice interfaces for nonconventional electronics is also explored. As examples, two-dimensionally confined electron-hole systems are predicted at the asymmetric interfaces in both Cs₂TiBr₆:LaAlO₃ and CsPbBr₃:LaAlO₃ superlattice structures. This finding, along with the optically active properties of PHs, may spark novel applications of light-electron interaction in perovskite systems. This work presents new opportunities for perovskite heteroepitaxial interfaces.